Preheat the oven to 350 degrees F (175 degrees C).

Warm oil in a skillet and add chicken. Season with onion powder. Cook until chicken is browned and just about done, 15 minutes. Add salt and pepper to taste.

Place chicken on a cookie sheet and top each breast with 1 to 2 slices of tomato and 2 to 3 slices of cheese.

Place in the preheated oven until cheese melts, 10 to 15 minutes. Remove from the oven, add 2 to 3 slices of avocado on top of each breast, and serve immediately.